Television and the like systems



Nov. 1, 1938. A. D. BLUMLEIN 2,134,851

TELEVISION AND THE LIKE SYSTEMS Filed Feb. 16, 1935 Patented Nov. 1,1938 TELEVISION AND THE LIKE SYSTEMS Britain Application February 16,1935, Serial No. 6,786 In Great Britain February 5, 1934 5 Claims.

The present invention relates to television and the like systems andmore particularly to systems employing for transmitting purposes acathode ray tube provided with a mosaic screen.

In one form of tube of this kind, an image to be transmitted isprojected on to a mosaic of small photo-electric elements insulated fromone another and from a conducting back-plate upon which they arecarried. The elements; are usually deposited upon an insulating layerprovided on the back-plate which will be referred to as the signalplate.

Within the tube are provided, in addition to the mosaic screen, a sourceof electrons, means for directing the electrons in a narrow beam uponthe screen, an anode for collecting electrons emitted by the screen anddeflecting means for causing the electron beam to scan the screen.

The deflecting means may be in the known form of coils or electrostaticplates which when supplied with electrical oscillations of saw-toothwave form cause the ray to scan the screen in parallel strips, eachscanning stroke being followed by a return stroke.

When the image is cast upon the screen, each element of the mosaic emitselectrons, and accordingly rises in potential, in proportion to thebrightness of its illumination and these electrons pass to the anode.What can be regarded as an electron image is thus formed on the "mosaicby the electric charges on the elements thereof. When the cathode raypasses over an element during the scanning operation, the charge thereonis neutralized thus bringing the element to a datum potential which isnegative with respect to the anode potential. When the ray leaves theelement the potential of the element again rises due to the emission ofphoto-electrons to the anode. The neutralizing charge given to eachelement by the cathode ray when it passes over the element is thereforedependent upon the amount of photo-electric discharge that has takenplace.

If a resistance be arranged in series between the signal plate and theanode of the device above described, there are developed across theresistance potential pulses which are representative of the brightnessof illumination of the successively scanned elements of the "mosaic.

So long as the average illumination of the mosaic screen remainsconstant, the average current flowing to the signal plate is zero. Whenthe cathode ray scans an element which has been brightly illuminated,the potential of the signal plate is made negative because of thearrival thereupon of charging electrons which are greater in number thanthe sum of the secondary electrons emitted owing to bombardment by theelectrons of the cathode ray beam and the total photo-electric electronsemitted.

When the cathode ray strikes an element which has not been illuminatedsince the last scan, no charge is given up by the ray, the number ofsecondary electrons emitted being equal to the number of electronsarriving; but at this instant the potential of the signal plate will bepositive owing to the steady photo-electric discharge occurring from allthe elements. Thus the signal generated across the resistance willrepresent true difierences in light and. shade between successivelyscanned elements but will not give any indication of the averageillumination.

In order that the average illumination may be transmitted, the D. 0.component of the signal may be established at some convenient point, forexample in the manner set forth in co-pending application Serial No.720,205. According to the method described in this prior application,the signal is caused at intervals to assume maximum or minimum valuesdiffering from picture black by a fixed amount but lying outside therange of picture signals and the D. C. component is inserted at anydesired point with reference to these recurring maxima or minima.

These recurrent maxima or minima may be established in the case of acathode ray system such as that above described by cutting off thecathode ray at the end of each scanning stroke so that there is nobombardment of the mosaic screen during the return stroke, that is tosay whilst the ray is being returned to one side of the screen after.scanning one strip of the image. Thus, during the return stroke, nocharge is delivered to the mosaic. The signal generated at this time onthe signal plate will not necessarily be zero, but will depart from zeroby an amount depending upon the total steady photoelectric currentarislng from electrons leaving the mosaic. The signal, however,corresponds to that generated, during a scanning stroke, when scanningan unilluminated element and therefore corresponds to full black.Consequently by shutting ofi the cathode ray beam during each returnstroke, recurrent signals of value corresponding to full black areobtained. A pulse in the "blacker than black sense may be superimposedupon each of the black signals and the recurrent minima so obtained maybe used in the manner described in the co-pending application abovereferred to, to re-establish the average picture intensity.

The pulses in the "blacker than black sense may also be used to controlthe generation of the saw-tooth scanning oscillations and, by applyingthem to a modulator electrode in the of the tube, during the firstscanning cycle after the change, will be quite diflerent from thatdescribed above. A sudden change of image brightness will cause a suddenchange in the photoelectric current from the mosaic and this change willcause a momentary additional current in the signal plate circuit. Theadditional current will be opposite in sense to that associated with thechange when considering changes from element to element with the averagebrightness constant. For example if the image be assumed to be quitedark, then the whole mosaic" will remain fully charged and no signalswill be transmitted during scanning, the conditions during the forward(or scanning) stroke being the same as those during the return strokewhen the ray is cut off.

If the mosaic now be suddenly illuminated, there will flow from thesignal plate a photoelectric current tending to make the signal platepositive by virtue of the resistance in series with it. The signal sogenerated would normally be associated with a black portion of thescreen and therefore this new signal, generated by a sudden increase inaverage brightness, is in the reverse direction to that normallyassociated with brighter illumination. In effect the signal representsblacker than black.

Immediately after the change of illumination, the cathode ray beam willnot deliver large charges to the mosaic because the increased light willnot have been active for long enough to discharge the "mosaic"appreciably. There will not therefore be any marked change of signalduring return strokes, when the ray is cut off, and the device forre-establishing D. C. will not operate immediately to change the D. 0.component of the signal in such a way as to represent the change inbrightness that has occurred. At the end of one scanning cycle (when theimage has been completely scanned once since the change) normaloperating conditions will have re-established themselves: duringscanning strokes the signal plate will be driven towards the negativecondition by the charge delivered by the cathode ray and during returnstrokes the signal plate will tend towards positive owing to the steadyphoto-electric current flowing.

Similar, but rather more dangerous conditions arise when theillumination of the mosaic is suddenly reduced. There may first beconsidered the worst case which occurs if the image which is beingtransmitted has a uniform intensity equal to the brightest whitetransmitted (referred to as full white) and if this image be replacedsuddenly by complete blackness. Before the change all the photo-electricelements are fully discharged by photo-electric current in the intervalsbetween successive scannings by the ray and the ray delivers the maximumcharge as it passes over the elements. At the end of each scanning lineand during the return stroke this charging terspersed with returns ofshort duration to a 1 level corresponding to full black, correspondingto the return strokes.

When the illumination is suddenly reduced to zero, the photo-electriccurrent component is completely stopped. The elements of the first I iewstrips scanned by the ray after the change are still fully discharged bythe previous illumination and in consequence there are delivered tothese elements by the ray charges which, as there is no photo-electricemission, make the signal plate highly negative. Also during the returnstrokes there is no photo-electric current to drive the signal platepositive so that no black signal, such as was transmitted before thechange, is generated. The result for the first instant after the changeof illumination is that instead of the full white" signals falling toblack" level, they rise to a value corresponding to nearly twice fullwhite" and the black signals between strips will rise to a levelcorresponding nearly to full white.

Thus the initial effect of reducing maximum brightness of image to zerois to generate a signal corresponding to almost twice full white, thatis to say the transient signals obtained are the reverse of what theyshould be.

The effect of these transient signals may be either to overload theapparatus, such as'amplifiers, radio frequency transmitter or receiversor,

if the apparatus is free from overload, to produce a very bright whiteflash at the receiver at just the moment at which the received pictureis intended to become black after being white. Further, the falsetransient signal is very likely to cause incorrect operation of anydevice serving to re-establish D. C. either at the transmitter orreceiver. Even if no means are provided in the system forre-establishing D. C., the transients produced will probably cause aserious upset of the apparatus.

It is an object of the present invention to provide means whereby thedisturbing effect of transients such as above described may be reducedor eliminated.

According to the present invention, a method of transmitting an image ofan object comprises the steps of forming an image of the object upon thephoto-electric surface of a screen, scanning the screen to generate inan output circuit picture currents representative of the light and shadeof the object, generating currents representative of the generalbrightness of the object and combining these latter currents with thepicture currents in such a way as to reduce the amplitude of reversepicture currents produced by transient changes in general brightness ofthe object.

Further according to the present invention there is provided a method oftelevision transm ssion in which an image to be transmitted is projectedupon a mosaic photo-electric screen and in which the potentials of themosaic elemcnts are periodically brought to a fixed value by scanning,characterized in that the amplitude of undesired impulses produced inthe picture signal circuit by sudden changes in the average intensity ofthe light falling upon said screen is reduced by developing andsuperimposing upon said undesired impulses, corrective impulses ofopposite sense to said undesired impulses.

The present invention also provides apparatus adapted for use incarrying out the methods above set forth.

The invention will be described with reference to the accompanyingdiagrammatic drawing, in which Fig. 1 shows one form of apparatus forcarrying the invention into effect, and Fig. 2 shows a modification ofthe arrangement of Fig. 1. Like parts in the two figures are given thesame references.

Referring to Fig. 1, a mosaic screen I is mounted within a bulbousportion of a closed glass envelope 2 having a cylindrical neck 3 inwhich are mounted an electron emitting cathode 3 and suitable electrondirecting electrodes 5. The anode is constituted by a silvering 6 of theinside of the envelope 2 in the neighbourhood of the junction betweenthe bulbous and cylindrical portions thereof. The screen I is arrangedat an angle of about to the axis of the cylindrical portion andtherefore to the mean direction of the cathode ray beam. The deflectionof the ray over the screen is effected by means of two pairs ofelectrcmagnet coils, of which only one pair, numbered l8, can be seen inthe figure, the coils being arranged to produce deflection of thecathode ray in two directions at right angles and being disposed outsidethe cylindrical part 3 of the tube; the coils are fed with currents ofsaw-tooth wave-form from a pair of suitable generators, of which one,numbered I9, is shown. Outside the envelope is arranged an opticalsystem 7 adapted to project an image of an object 8 upon the screen I.The optical axis of the optical system is arranged to be normal to thescreen. An arrangement of this kind is described for example by V. K.Zworykin in an article entitled "Television with cathode-ray tubesappearing in the Journal of The Institution of Electrical Engineers,vol. 73, and commencing on page 437.

The signal plate of the mosaic screen I may be connected to the grid ofan amplifying valve 9 and through a suitable resistance I 0 and biasbattery H or other source of E. M. F. to earth and to the cathode of thevalve 9. The cathode of the valve 9 is connected to the anode of thecathode ray tube which is maintained at a suitable positive potentialrelative to the cathode of the cathode ray tube by means of a battery orother source 12.

In order to neutralize the transient effects above described, anauxiliary photo-electric cell It is arranged with its anode it connectedto a tapping point on the resistance l0 and its cathode 20 connectedthrough a suitable bias battery IE or other source of E. M. F. to earth.

The auxiliary photo-electric cell I3 is arranged in a box It which isclosed excepting for an aperture 11, which may be the aperture in anadjustable diaphragm, and through which the cell l3 receives light fromthe whole of the object 8. The interior of the box may be blackened. Thesensitivity of this cell l3 and the adjustment of the tapping point onthe resistance in are arranged to be such that, with sudden changes ofillumination of the object 8, the transient voltages developed acrossthe resistance ill by the signal plate of the mosaic screen I and by theauxiliary cell I3 are substantially equal and opposite.

lliary cell l3 whatever may be the brightness of illumination of theobject.

The relative sensitivities of the photo-cell constituted by the "mosaic"screen and signal plate and the auxiliary cell may be adjusted in anyother known or suitable manner, for example by adjusting the relativeamounts of light falling upon the two devices with the aid of suitableadjustable diaphragm stops. The correct adjustment may be determined bycausing the brightness of illumination of the object to vary rapidly andso adjusting the relative sensitivities that the signals obtainedcorrectly represent the changes in illumination.

For very rapid alternation of illumination, such as an illuminationwhich changes at a frequency much greater than the frame frequency, noappreciable output signal should be produced. No transient having aduration less than the time of one complete scanning cycle can beaccurately transmitted, although a very intense transient such as aelectric spark discharge may be transmitted owing to it causing a rapiddischarge of the mosaic elements and being therefore transmitted in theform of a flash of lower intensity and longer duration.

When transmitting pictures from film, the

successive frames being projected one at a time upon the mosaic, it ispossible to move the film forwards during the scanning of the "mosaic."If this is done with an uncompensated system, however, the sudden changein illumination of the mosaic owing to the interposition of a shutter oreven, if no shutter is used, owing to the black strip between frames,produces an undesired transient in the middle of the picture signals.With the aid of the compensating arrangement of the present invention,however, the film may be moved forward during the scanning cycle withoutthe generation of a harmful transient and more time is thereforeavailable for moving the film than the interval between the transmissionof two successive frames and these intervals may be made as short aspracticable.

It is not necessary to focus an image of the object 8 upon the auxiliarycell l3, in fact this is usually undesirable because the cathode of theauxiliary cell may not be of uniform sensitivity over its surface.Referring now to Fig. 2, which shows a modification of the arrangementof Fig. 1, an image of an object 8 (which may be a cinematograph film)to be transmitted is projected by the aid of light source 2| on to themosaic screen I of a cathode ray tube 2, 3 which contains within itsenvelope the electrodes M and 20 of an auxiliary photo-electric cell l3.

An image of the object 8 is also thrown on to the cathode 20 of the celll3 by the aid of a separate light source 22 and the optical system 21.The output of the mosaic" cell is amplified in amplifiers 9 and 23,while the output from the auxiliary cell I3 is separately amplified inamplifier 24. The outputs from amplifiers 23 and 26 are mixed togetherin opposition at 25, and the combined output is fed to a transmitter 2%.

It is desirable in the arrangement shown in Fig. 2 either that the timeconstants of the coupling or couplings in the amplifier 9, 23 and in theamplifier 24 which determine the lower cut-oil frequency, be madegreater than 75th of a second (where 25 pictures are transmitted persecond) or that the time constants of the two amplifiers 9, 23 and 24should be made substantially equal to one another.

Although the invention has been described in some detail with referenceto a. particular form of cathode ray tube, it is also applicable toother forms of mosaic type tubes which tend, on a change of illuminationof the object, to generate a transient having a sense opposite to thatnormally corresponding to that change of illumination.

Further the invention is not limited to systems in which the D. C.component is retained or re-inserted before transmission of the signal.It can also be applied to systems in which the D. 0. component issuppressed or at least not utilized.

I claim:

1. Apparatus for transmitting an image of an object, said apparatuscomprising a mosaic screen of mutually insulated photo-electricelements, optical means for directing light from said object along afirst optical path to form an image of said object upon said screen,scanning means for periodically bringing said elements to a fixedpotential by scanning, an output circuit associated with said screen forreceiving picture signals generated during said scanning, an auxiliarylight sensitive device, means for projecting light from said objectalong a second optical path distinct from said first path upon saidauxiliary device to generate therein auxiliary signals representative ofthe general brightness of said object, and means for feeding saidauxiliary signals into said output circuit directly in opposition tosaid picture signals the coupling means between the auxiliary signalgenerating means, and the video amplifier having a time constantsubstantially equal each to the other.

2. Apparatus for transmitting an image of an object, said apparatuscomprising a mosaic screen of mutually insulated photo-electricelements, means for forming an image of said object upon said screen,scanning means for periodically bringing said elements to a fixedpotential by scanning, an output circuit associated with said screen forreceiving picture signals generated during said scanning, an auxiliarylight sensitive device, means for projecting light from said object uponsaid auxiliary device to generate auxiliary signals representative ofthe general brightness of said object, means for adjusting the amount ofsaid light and means for feeding said auxiliary signals directly intosaid output circuit in opposition to said picture signals the couplingmeans between the auxiliary signal generating means, and the videoamplifier having a time constant substantially equal each to the other.

3. Apparatus for transmitting an image of an object, said apparatuscomprising a mosaic screen of mutually insulated photo-electricelements, scanning means for periodically bringing said elements to afixed potential by scanning, an auxiliary light sensitive device, anenvelope containing said screen, said scanning means and said device,means for directing light from said object along a first path to form animage of said object upon said screen, an output circuit associated withsaid screen for receiving picture'signals generated during saidscanning, means for directing light from said object, along a secondpath different from said first path, to fall upon said auxiliary deviceand thereby to generate auxiliary signals representative of the generalbrightness of said object, and means for feeding said auxiliary signalsdirectly into said output circuit in opposition to said picture signalsthe coupling means between the auxiliary signal generating means, andthe video amplifier having a time constant substantially equal each tothe other.

4. Apparatus for transmitting an image of an object, said apparatuscomprising a mosaic screen of mutually insulated photo-electricelements, means for continuously maintaining an image of said objectupon said screen, scanning means for periodically bringing said elementsto a fixed potential by scanning, an output circuit directly associatedwith said screen for receiving picture signals generated during saidscanning, an auxiliary light sensitive device, means for projectinglight directly from said object upon said auxiliary device to generateauxiliary signals representative of the general brightness of saidobject, and means for feeding said auxiliary signals directly into saidoutput circuit in opposition to said picture signals the coupling meansbetween the auxiliary signal generating means, and the video amplifierhaving a time constant substantially equal each to the other.

5. Apparatus for transmitting an image of an object, said apparatuscomprising a mosaic screen of mutually insulated photo-electricelements, optical means for directing light from said object along afirst optical path to form an image of said object upon said screen,scanning means for bringing said elements to a fixed potential byscanning, an output circuit associated with said screen for receivingpicture signals generated during said scanning, an auxiliary lightsensitive device, means for projecting light from said object along asecond optical path distinct from said first path upon said auxiliarydevice to generate auxiliary signals representative of the generalbrightness of the object, and means for feeding said auxiliary signalsinto said output circuit directly in opposition to said picture signals.

ALAN DOWER BLUMLEIN.

